20 Mind-Blowing Easter Eggs Hidden on Famous Albums | Cracked.com.

On the Sgt Peppers one, I’m assuming that they are referring to 18 kilohertz.

Which is within the standard range for human hearing, which tops out somewhere around 20-22khz, though the upper end tends to go first as you age.  Their legions of teenage fans would have almost certainly been able to hear that tone.

Some of Crackeds photoplasties don’t depend on reality, but those that do like this really should go through cursory fact checking before being included.